Various non-contact proximity sensors have been developed for use in determining gauge from a fast moving survey vehicle. These sensors include capacitive, magnetic and electro-optical sensors. Exemplary of these devices are those set forth in Wallen U.S. Pat. No. 3,517,307 and in Wilmorth U.S. Pat. No. 3,864,039, each of which are sensitive to structure at the running surface or at its juncture with the gauge surface where errors are introduced by the aforementioned overhanging lip. In order to measure the gauge at the five-eighths inch point, hereinafter referred to as the gauge point, a capacitive or magnetic sensor must be mounted below a plane including the top or running surface of the rail. However, when so located the sensor suffers from poor sensitivity at large air gaps and from distortion of the measurement by rail components that are extraneous to gauge, e.g., the rail web and base. While this problem can be overcome by a servo-driven gauge sensor carriage which maintains a small air gap between sensor and rail head regardless of gauge deviations, the moving parts of the servo present longevity problems in the rugged rail environment. Moreover, such sensors are vulnerable to damage in areas where certain track structures or ancillary structures protrude into the area between the rail heads, e.g., at grade crossings, rail crossings, switches, and guard rails, and have required provisions for elevating the sensors while the survey vehicle traverses such structures. These provisions in turn require the services of an alert operator or possibly an additional sensor and control system, and are incapable of maintaining measurement during passage over such objects.
In the field of non-contact position sensing generally, numerous electro-optical systems have been developed which employ a projector for illuminating a point on a sensed surface and an optical system for observing the illuminated point. Deviations in the distance between the sensor and the illuminated point cause movement of the observed image which is detected by an appropriate photosensitive detector. Examples of such systems are found in Mansfield U.S. Pat. No. 3,821,558; Svetlichny U.S. Pat. No. 3,633,010; Hosterman U.S. Pat. Nos. 3,589,815; Bailey 3,016,464; and Gunther et al U.S. Pat. No. 3,137,756. The sensing in each of the foregoing patent disclosures takes place in a single plane defined by the projection and the sensing optical paths, and the sensed positional deviations take place within this same plane. Hence, if such an electro-optical system were employed in sensing the gauge point of railroad rails, it too would have to be placed below the plane defined by the running surfaces of the two rails. In this position, it is subject to the aforementioned interference from structural obstructions.
An electro-optical measurement system disposed wholly above the plane of the rail surfaces is disclosed in the aforementioned Wilmarth U.S. Pat. No. 3,864,039. This patent discloses a system which relies on the position of a shadow boundary as representative of the position of the inner surface of the rail head, and does not represent effective gauge as defined by the aforementioned DOT/FRA standard.
Finally, an electro-optical measurement system disposed above some points of measurement is disclosed in Bess U.S. Pat. No. 3,393,600, but this system does not confine its point of measurement to a single plane.